Estimation system and reception node

ABSTRACT

According to an embodiment, an estimation system includes a first node, a second node, and an estimator. The estimator estimates, based on a propagation time of a wave, (i) a distance from a first transmission source having transmitted the wave to the first node or (ii) a location of the first transmission source or the first node. The first node includes a wave receiver and a first wireless communicator. The wave receiver receives the wave. The first wireless communicator incorporates a first timer, synchronizes the first timer with a second timer built in the second node via wireless communication with the second node, acquires reception time information indicative of a point in time of reception of the wave from the first timer, and outputs the reception time information to the estimator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2014-176091, filed Aug. 29, 2014, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments relate to estimation of a distance or a location based onwave propagation time.

BACKGROUND

GPS (Global Positioning System) is conventionally utilized to estimatelocation information for car navigation systems, smartphones, and thelike. However, in an environment where radio waves from satellites aredifficult to receive (for example, in a closed environment), estimatinglocation information utilizing GPS is difficult.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a location estimation systemaccording to a first embodiment;

FIG. 2 is a block diagram illustrating a wave reception node in FIG. 1;

FIG. 3 is a flowchart illustrating operations of a communicationcontroller in FIG. 2;

FIG. 4 is a block diagram depicting a modification of the system in FIG.1;

FIG. 5 is a block diagram depicting a modification of the system in FIG.1;

FIG. 6 is a block diagram depicting a modification of the system in FIG.1;

FIG. 7 is a block diagram illustrating a wave reception node included ina location estimation system according to a second embodiment;

FIG. 8 is a flowchart illustrating operations of a communicationcontroller in FIG. 7;

FIG. 9 is a block diagram illustrating a location estimation systemaccording to a third embodiment;

FIG. 10 is a block diagram illustrating a wave transmission andreception node in FIG. 9;

FIG. 11 is a flowchart illustrating operations of a communicationcontroller in FIG. 10;

FIG. 12 is a block diagram illustrating a location estimation systemaccording to a fourth embodiment;

FIG. 13 is a block diagram illustrating a wave transmission node in FIG.12;

FIG. 14 is a block diagram illustrating a wave reception node in FIG.12;

FIG. 15 is a block diagram illustrating a location estimation systemaccording to a fifth embodiment;

FIG. 16 is a block diagram illustrating a wave reception node in FIG.15;

FIG. 17 is a sequence diagram illustrating operations of the locationestimation system in FIG. 12; and

FIG. 18 is a sequence diagram illustrating operations of the locationestimation system in FIG. 15.

DETAILED DESCRIPTION

Embodiments will be described below with reference to the drawings.

According to an embodiment, an estimation system includes a node groupand an estimator. The node group includes a first node receiving a firstwave and a second node communicating wirelessly with the first node. Theestimator estimates, based on a propagation time of the first wave, (i)a distance from a first transmission source having transmitted the firstwave to the first node or (ii) a location of the first transmissionsource or the first node. The first node includes a first wave receiverand a first wireless communicator. The first wave receiver receives thefirst wave. The first wireless communicator incorporates a first timer,synchronizes the first timer with a second timer built in the secondnode via wireless communication with the second node, acquires a firstpiece of reception time information indicative of a point in time ofreception of the first wave from the first timer, and outputs the firstpiece of reception time information to the estimator.

According to another embodiment, an estimation system includes atransmission node group, a reception node and an estimator. Thetransmission node group includes a first transmission node, a secondtransmission node, a third transmission node, and a fourth transmissionnode that transmit a first wave, a second wave, a third wave, and afourth wave and communicates wirelessly with one another. The receptionnode receives the first wave, the second wave, the third wave, and thefourth wave. The estimator estimates a location of the reception nodebased on propagation times of the first wave, the second wave, the thirdwave, and the fourth wave. The first transmission node includes a firstwave transmitter and a first wireless communicator. The first wavetransmitter transmits the first wave. The first wireless communicatorincorporates a first timer, synchronizes the first timer with a secondtimer, a third timer, and a fourth timer built in the secondtransmission node, the third transmission node, and the fourthtransmission node via wireless communication with the secondtransmission node, the third transmission node, and the fourthtransmission node, and controls a timing when the first wave transmittertransmits the first wave based on a timer value of the first timer. Thesecond transmission node includes a second wave transmitter and awireless communicator. The second wave transmitter transmits the secondwave. The second wireless communicator incorporates the second timer,synchronizes the second timer with the first timer, the third timer, andthe fourth timer via wireless communication with the first transmissionnode, the third transmission node, and the fourth transmission node, andcontrols a timing when the second wave transmitter transmits the secondwave based on a timer value of the second timer. The third transmissionnode includes a third wave transmitter and a third wirelesscommunicator. The third wave transmitter transmits the third wave. Thethird wireless communicator incorporates the third timer, synchronizesthe third timer with the first timer, the second timer, and the fourthtimer via wireless communication with the first transmission node, thesecond transmission node, and the fourth transmission node, and controlsa timing when the third wave transmitter transmits the third wave basedon a timer value of the third timer. The fourth transmission nodeincludes a fourth wave transmitter and a fourth wireless communicator.The fourth wave transmitter transmits the fourth wave. The fourthwireless communicator incorporates the fourth timer, synchronizes thefourth timer with the first timer, the second timer, and the third timervia wireless communication with the first transmission node, the secondtransmission node, and the third transmission node, and controls atiming when the fourth wave transmitter transmits the fourth wave basedon a timer value of the fourth timer. The reception node notifies theestimator that the reception node has received the first wave, thesecond wave, and the third wave. The estimator calculates propagationtime differences between a propagation time of the first wave from thefirst transmission node to the reception node and propagation times ofthe second wave, the third wave, and the fourth wave from the secondtransmission node, the third transmission node, and the fourthtransmission node to the reception node based on points in time ofreception of notifications that the first wave, the second wave, thethird wave, and the fourth wave, respectively, have been received fromthe reception node, to estimate the location of the reception node basedon the propagation time differences.

According to another embodiment, a reception node includes a wavereceiver and a wireless communicator. The wave receiver receives a soundwave. The wireless communicator incorporates a first timer, synchronizesthe first timer with a second timer built in another node via wirelesscommunication with the other node, and acquires reception timeinformation indicative of a point in time of reception of the sound wavefrom the first timer.

Elements identical or similar to described elements are denoted byidentical or similar elements, and duplicate descriptions are basicallyomitted. For example, when a plurality of identical or similar elementsis present, a common reference numeral may be used in order to avoiddistinguishing the elements from one another in the description orbranch numbers may be used in addition to the common reference numeralin order to distinguish the elements from one another in thedescription.

First Embodiment

As illustrated in FIG. 1, a location estimation system according to afirst embodiment includes four wave reception nodes 100-1, 100-2, 100-3,and 100-4, a wave transmission source 130, and a location estimator 140.The total number of wave reception nodes 100 may be five or more.

The wave transmission source 130 can transmit a wave. The wave may havea predetermined characteristic as described below so as to bedistinguishable from other waves. A sound wave (including an ultrasonicwave) is hereinafter utilized as the wave. However, any other wave suchas an electromagnetic wave or an oscillatory wave may be utilized. Thewave transmission source 130 may be an apparatus such as a robot with aloudspeaker or a living organism such as an animal or a human being.When the wave transmission source 130 is a living organism, voices,calls, or vibration from the living organism corresponds to the wave.

The wave reception nodes 100-1, 100-2, 100-3, and 100-4 receive a wave.Upon detecting that the received wave has, for example, theabove-described characteristic, the wave reception nodes 100-1, 100-2,100-3, and 100-4 acquire information indicative of the point in time ofreception of the wave (hereinafter referred to as reception timeinformation). The reception time information may be, for example, atimer value. In order to reduce the magnitude of location estimationerrors, timers referenced by the wave reception nodes 100-1, 100-2,100-3, and 100-4 preferably synchronize accurately with one another. Thereception time information is collected by the location estimator 140.

Specifically, the wave reception node 100 includes a wave receiver 110and a wireless communicator 120. The wave receiver 110 receives a wavetransmitted by the wave transmission source 130. Upon detecting that thereceived wave has the above-described characteristic, the wave receiver110 notifies the wireless communicator 120 that the desired wave hasbeen received.

Upon receiving the notification from the wave receiver 110, the wirelesscommunicator 120 accesses a built-in timer to acquire the reception timeinformation. The wireless communicator 120 outputs the reception timeinformation to the location estimator 140. The location estimator 140may be connected to any one of the wave reception nodes 100-1, 100-2,100-3, and 100-4 in a wired or wireless manner. When directly connectedto the location estimator 140, the wireless communicator 120 outputs thereception time information directly to the location estimator 140. Onthe other hand, when not directly connected to the location estimator140, the wireless communicator 120 outputs the reception timeinformation to the location estimator 140 via another wave receptionnode 100.

A specific example of the wave reception node 100 is depicted in FIG. 2.The wave receiver 110 includes a microphone 111 and a detector 112. Thewireless communicator 120 includes a communication controller 121, asynchronization timer 122, a wireless receiver 123, and a wirelesstransmitter 124.

The microphone 111 receives and converts a sound wave into an electricsignal. The microphone 111 outputs the electric signal to the detector112.

The detector 112 receives the electric signal from the microphone 111,and checks whether or not the electric signal has a predeterminedcharacteristic. Upon detecting that the electric signal has apredetermined characteristic, the detector 112 notifies thecommunication controller 121 that the desired wave has been received.For example, the sound wave from the wave transmission source 130 mayhave a predetermined sound pressure level, a predetermined timewaveform, or a predetermined frequency component as a predeterminedcharacteristic.

The communication controller 121 controls the synchronization timer 122,the wireless receiver 123, and the wireless transmitter 124. Forexample, upon receiving the notification from the detector 112, thecommunication controller 121 acquires a timer value from thesynchronization timer 122. The timer value corresponds to the receptiontime information. The communication controller 121 outputs the receptiontime information to the location estimator 140.

Specifically, when directly connected to the location estimator 140, thecommunication controller 121 outputs the reception time informationdirectly to the location estimator 140. On the other hand, when notdirectly connected to the location estimator 140, the communicationcontroller 121 allows the wireless transmitter 124 to transmit thereception time information to the location estimator 140 or another wavereception node 100. When the wireless receiver 123 receives thereception time information from another wave reception node 100, thecommunication controller 121 similarly outputs the reception timeinformation to the location estimator 140.

Moreover, the communication controller 121 executes a synchronizationprocess for synchronizing the synchronization timer 122 with thesynchronization timer 122 built in another wave reception node 100. Forexample, the communication controller 121 may allow the wirelesstransmitter 124 to transmit the timer value of the synchronization timer122 to another wave reception node 100 or may correct the timer value ofthe synchronization timer 122 using the timer value in another wavereception node 100 received by the wireless receiver 123.

Specifically, the communication controller 121 may operate asillustrated in FIG. 3. First, the communication controller 121 continuesto wait for a notification from the detector 112 (step S101). Then, uponreceiving a notification from the detector 112, the communicationcontroller 121 acquires the timer value of the synchronization timer 122(step S102).

When directly connected to the location estimator 140, the communicationcontroller 121 outputs the timer value acquired in step S102 directly tothe location estimator 140 (step S103 and step S104). On the other hand,when not directly connected to the location estimator 140, thecommunication controller 121 allows the wireless transmitter 124 totransmit the timer value acquired in step S102 to the location estimator140 or another wave reception node 100 (step S103 and step S105).

The synchronization timer 122 obtains the timer value by performing acount-up operation in synchronization with a clock signal. Thesynchronization timer 122 is controlled by the communication controller121 so as to synchronize with the synchronization timer 122 built inanother wave reception node 100. For example, given that the wirelesscommunicator 120 corresponds to wireless LAN equipment compliant withIEEE 802.11, the synchronization timer 122 may be implemented by a TSF(Timing Synchronization Function) timer. The synchronization process forthe TSF timer varies depending on the network configuration(infrastructure mode or ad hoc mode) but enables accuratesynchronization in any network configurations. For example, according toIEEE 802.11, the synchronization error between TSF timers is at mostseveral microseconds. Furthermore, even when the wireless communicator120 is equipment compliant with another wireless communication standardsuch as IEEE 802.15.1 or IEEE 802.15.4, if the wireless communicator 120incorporates a similar timer, the timer can be used.

The wireless receiver 123 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 123receives the reception time information and information needed for thesynchronization process (for example, the timer value of thesynchronization timer 122 built in another wave reception node 100) fromanother wave reception node 100 in the form of wireless signals.

The wireless transmitter 124 transmits various types of information inthe form of wireless signals. Specifically, the wireless transmitter 124transmits the reception time information and information needed for thesynchronization process (for example, the timer value of thesynchronization timer 122) to another wave reception node 100, thelocation estimator 140, and the like in the form of wireless signals.The wireless transmitter 124 may transmit the identifier of the wavereception node 100 together with the reception time information. Forexample, given that the wireless communicator 120 is wireless LANequipment compliant with IEEE 802.11, the identifier may be a MAC (MediaAccess Control) address.

The location estimator 140 collects the reception time information fromthe wave reception nodes 100-1, 100-2, 100-3, and 100-4. The locationestimator 140 estimates the location of the wave transmission source 130based on the collected reception time information, the velocity of thewave, and the location of each wave reception node 100. The locationestimator 140 can reference, for example, the identifier and location ofeach wave reception node 100 and the velocity of the wave. Theidentifier and location of each wave reception node 100 may beregistered when the wave reception node 100 is installed.

When the location of the wave transmission source 130 is represented as(x, y, z), the location of the wave reception node 100-i (i is aninteger of at least 1 and at most 4) is represented as (x_(i), y_(i),z_(i)), the time of wave propagation from the and reception node fromthe wave transmission source 130 to the wave reception node 100-i isrepresented as t_(i), and the velocity of the wave (for example, a soundwave) is represented as v, Expression (1) holds true.

$\begin{matrix}{{vt}_{i} = \sqrt{\left( {x - x_{i}} \right)^{2} + \left( {y - y_{i}} \right)^{2} + \left( {z - z_{i}} \right)^{2}}} & (1)\end{matrix}$

Moreover, for i >1, when the difference between the reception timeinformation collected from the wave reception node 100-i and thereception time information collected from the wave reception node 100-1is represented as Δt_(i), Expression (2) holds true.

t _(i) =t ₁ +Δt _(i)  (2)

Based on Expressions (1) and (2), the following linear simultaneousequation with four unknowns holds true for unknowns x, y, x, and t_(i).

$\begin{matrix}\left\{ \begin{matrix}{{vt}_{1} = \sqrt{\left( {x - x_{1}} \right)^{2} + \left( {y - y_{1}} \right)^{2} + \left( {z - z_{1}} \right)^{2}}} \\{{v\left( {t_{1} + {\Delta \; t_{2}}} \right)} = \sqrt{\left( {x - x_{2}} \right)^{2} + \left( {y - y_{2}} \right)^{2} + \left( {z - z_{2}} \right)^{2}}} \\{{v\left( {t_{1} + {\Delta \; t_{3}}} \right)} = \sqrt{\left( {x - x_{3}} \right)^{2} + \left( {y - y_{3}} \right)^{2} + \left( {z - z_{3}} \right)^{2}}} \\{{v\left( {t_{1} + {\Delta \; t_{2}}} \right)} = \sqrt{\left( {x - x_{4}} \right)^{2} + \left( {y - y_{4}} \right)^{2} + \left( {z - z_{4}} \right)^{2}}}\end{matrix} \right. & (3)\end{matrix}$

In Expression (3), v, x₁, y₁ z₁, x₂, y₂, z₂, x₃, y₃, z₃, x₄, y₄, and z₄are known to the location estimator 140. Furthermore, the locationestimator 140 can calculate Δt₂, Δt₃, and Δt₄ based on the receptiontime information collected from the wave reception nodes 100-1, 100-2,100-3, and 100-4. Hence, the location estimator 140 can calculate thelocation (x, y, z) of the wave transmission source 130 by solvingExpression (3). Moreover, the location estimator 140 can calculate thedistance from the wave reception node 100-i to the wave transmissionsource 130 by, for example, substituting x, y, and z into the right sideof Expression (1).

In FIG. 1, the wireless communicators 120-1, 120-2, 120-3, and 120-4 aredepicted to form a peer-to-peer network in, for example, an ad hoc modein IEEE 802.11. However, the wireless communicators 120-1, 120-2, 120-3,and 120-4 may form a star network in, for example, an infrastructuremode in IEEE 802.11.

For example, the location estimation system in FIG. 1 may be modified asillustrated in FIG. 4. The location estimation system in FIG. 4 includesthe location estimator 140, the wave transmission source 130, and fourwave reception nodes 200-1, 200-2, 200-3, and 200-4. The total number ofwave reception nodes 200 may be five or more.

A wireless communicator 220-1 included in the wave reception node 200-1functions as a master station corresponding to a hub of the star network(for example, an AP (Access Point) in IEEE 802.11). On the other hand,wireless communicators 220-2, 220-3, and 220-4 included in the wavereception nodes 200-2, 200-3, and 200-4 function as slave stations (forexample, STA (Stations) in IEEE 802.11).

In general, in the star network, the master station can communicatedirectly with the slave stations. On the other hand, each of the slavestations can communicate with the other slave stations via the masterstation. The example in FIG. 4 where the location estimator 140 isconnected to the master station enables a reduction in traffic for thereception time information compared to a case where the locationestimator 140 is connected to the slave station.

It should be noted that the master station undergoes a heaviercommunication load than the slave stations. Thus, a timing when thetimer value (for example, the reception time information) of thebuilt-in timer may be delayed, which degrades location estimationaccuracy. Hence, the master station may be prepared separately from thewave reception node as illustrated in FIG. 5. The location estimationsystem in FIG. 5 includes the location estimator 140, the wavetransmission source 130, the four wave reception nodes 200-1, 200-2,200-3, and 200-4, and a wireless communication apparatus 350. The totalnumber of wave reception nodes 200 may be five or more.

The wireless communication apparatus 350 functions as a master stationcorresponding to the hub of the star network (for example, an AP (AccessPoint) in IEEE 802.11). On the other hand, the wireless communicators220 included in each wave reception node 200 function as slave stations(for example, STA (Stations) in IEEE 802.11).

The wireless communication apparatus 350 serving as the master stationcollects the reception time information from the wave reception nodes200 and outputs the reception time information to the location estimator140. Each wireless communicator 220 needs to be designed to have afunction to acquire the timer value of the built-in timer. However, thewireless communication apparatus 350 needs no such function. Thus, thewireless communication apparatus 350 can be implemented even by ageneral-purpose AP as long as the AP meets basic usage conditions, forexample, the number of simultaneous connections.

Moreover, the location estimator 140 need not be an apparatusindependent of the wireless communicator (for example, PC (PersonalComputer)). For example, as depicted in FIG. 6, the location estimator140 may be incorporated into a wireless communicator 420-1 (masterstation) of a wave reception node 400. When the location estimator 140is incorporated into the wireless communicator 420-1, the number ofapparatuses included in the location estimation system according to thepresent embodiment can be reduced, facilitating installation of thelocation estimation system. The location estimator 140 may beincorporated into the slave station instead of the master station.

As described above, the location estimation system according to thefirst embodiment includes the four or more wave reception nodes, and thewireless communicators included in the respective wave reception nodesform a wireless communication network. Each wave reception node acquiresthe reception time information on the wave transmitted by the wavetransmission source, from the timer built in the wireless communicator,and outputs the reception time information to the location estimator.

The location estimator estimates the location of the wave transmissionsource based on the collected reception time information, the velocityof the wave, and the location of each wave reception node. The timerssynchronize accurately with one another among the wave reception nodes,and thus, the location estimator can collect accurate wave propagationtime differences among the wave reception nodes to accurately estimatethe location of the wave transmission source 130.

Furthermore, the wave reception node can acquire the reception timeinformation from the timer built in the wireless communicator in thewave reception node. That is, the wave reception node need notseparately have, for example, hardware or software configured to executea time synchronization process based on the NTP (Network Time Protocol).The location estimation system needs no special apparatus for timesynchronization such as an NTP server. In addition, the wave receptionnodes are wirelessly connected together. Thus, no communication cableneeds to be laid, and the wireless communicator included in the wavereception node may be implemented using, for example, general-purposewireless LAN equipment compliant with IEEE 802.11. Hence, the locationestimation system can be inexpensively, simply, and compactlyconstructed, and the apparatuses included in the location estimationsystem can be easily installed.

Second Embodiment

A location estimation system according to a second embodimentcorresponds to the location estimation system in FIG. 1 in which thelocation estimator 140 is replaced with a location estimator 540 and inwhich each wave reception node 100 is replaced with a wave receptionnode 500. The total number of wave reception nodes 500 may be five ormore.

The wave reception node 500 receives a wave to acquire reception timeinformation. Specifically, the wave reception node 500 includes a wavereceiver 510 and a wireless communicator 520. The wave receiver 510receives a wave transmitted by a wave transmission source 130. The wavereceiver 510 converts the received wave into digital data (hereinafterreferred to as wave data) and outputs the wave data to the wirelesscommunicator 520.

Upon receiving a notification from the wave receiver 510, the wirelesscommunicator 520 accesses a built-in timer to acquire the reception timeinformation. The wireless communicator 520 outputs the reception timeinformation and the wave data to the location estimator 540. Thelocation estimator 540 may be connected to any one of the wave receptionnodes 500-1, 500-2, 500-3, and 500-4 in a wired or wireless manner. Whendirectly connected to the location estimator 540, the wirelesscommunicator 520 outputs the reception time information and the wavedata directly to the location estimator 540. On the other hand, when notdirectly connected to the location estimator 540, the wirelesscommunicator 520 outputs the reception time information and the wavedata to the location estimator 540 via another wave reception node 500.

A specific example of the wave reception node 500 is depicted in FIG. 7.The wave receiver 510 includes a microphone 111 and a converter 512. Thewireless communicator 520 includes a communication controller 521, asynchronization timer 122, a wireless receiver 523, and a wirelesstransmitter 524.

The converter 512 receives an electric signal from the microphone 111and converts the electric signal into wave data. The converter 512outputs the wave data to the communication controller 521. The converter512 may sequentially convert a sound wave into frames at predeterminedintervals to generate wave data in a sound frame format.

The communication controller 521 controls the synchronization timer 122,the wireless receiver 523, and the wireless transmitter 524. Forexample, upon receiving the wave data, the communication controller 521acquires the timer value of the synchronization timer 122. The timervalue corresponds to information indicative of the point in time ofreception of a sound wave by the wave reception node 500 from the wavetransmission source 130. The communication controller 521 outputs thereception time information and the wave data to the location estimator540.

Specifically, when directly connected to the location estimator 540, thecommunication controller 521 outputs the reception time information andthe wave data directly to the location estimator 540. On the other hand,when not directly connected to the location estimator 540, thecommunication controller 521 allows the wireless transmitter 524 totransmit the reception time information and the wave data to thelocation estimator 540 or another wave reception node 500. When thewireless receiver 523 receives the reception time information and thewave data from another wave reception node 500, the communicationcontroller 521 similarly outputs the reception time information and thewave data to the location estimator 540.

Moreover, the communication controller 521 executes a synchronizationprocess for synchronizing the synchronization timer 122 with thesynchronization timer 122 built in another wave reception node 500. Forexample, the communication controller 521 may allow the wirelesstransmitter 524 to transmit the timer value of the synchronization timer122 to another wave reception node 500 or may correct the timer value ofthe synchronization timer 122 using the timer value in another wavereception node 500 received by the wireless receiver 523.

Specifically, the communication controller 521 may operate asillustrated in FIG. 8. First, the communication controller 521 continuesto wait for the wave data from the converter 512 (step S201). Then, uponreceiving the wave data from the converter 512, the communicationcontroller 521 acquires the timer value of the synchronization timer 122(step S202).

When directly connected to the location estimator 540, the communicationcontroller 521 outputs the timer value and wave data acquired in stepS202 directly to the location estimator 540 (step S203 and step S204).On the other hand, when not directly connected to the location estimator540, the communication controller 521 allows the wireless transmitter524 to transmit the timer value and wave data acquired in step S202 tothe location estimator 140 or another wave reception node 500 (step S203and step S205).

The wireless receiver 523 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 523receives the reception time information, the wave data, and informationneeded for the synchronization process (for example, the timer value ofthe synchronization timer 122 built in another wave reception node 500)from another wave reception node 100 in the form of wireless signals.

The wireless transmitter 524 transmits various types of information inthe form of wireless signals. Specifically, the wireless transmitter 524transmits the reception time information, the wave data, and informationneeded for the synchronization process (for example, the timer value ofthe synchronization timer 122) to another wave reception node 500, thelocation estimator 540, and the like in the form of wireless signals.The wireless transmitter 524 may transmit the identifier of the wavereception node 500 together with the reception time information and thewave data.

The location estimator 540 collects the reception time information andthe wave data from the wave reception nodes 500-1, 500-2, 500-3, and500-4. As described above, the sound wave from the wave transmissionsource 130 may have, for example, predetermined sound pressure level, apredetermined time waveform, or a predetermined frequency component as apredetermined characteristic in order to be distinguished from othersound waves. Upon detecting that the collected wave data has thepredetermined characteristic, the location estimator 540 estimates thelocation of the wave transmission source 130 based on the reception timeinformation corresponding to the wave data, the velocity of the wave,and the location of each wave reception node 500. The location estimator540 may neglect the wave data and the corresponding reception timeinformation when the collected wave data does not include thepredetermined characteristic. The location estimator 540 can reference,for example, the identifier and location of each wireless communicator520 and the velocity of the wave. The identifier and location of eachwireless communicator 520 may be registered when the wave reception node500 is installed.

The wireless communicators 520-1, 520-2, 520-3, and 520-4 may form astar network instead of a peer-to-peer network. Furthermore, a masterstation may be provided independently of the wave reception node 500.Moreover, the location estimator 540 may be incorporated into any one ofthe wireless communicators 520.

As described above, the location estimation system according to thesecond embodiment is different from the location estimation systemaccording to the first embodiment in that the process in which, insteadof each wave reception node, the location estimator intensively checkswhether or not the wave received by the wave reception node has thepredetermined characteristic. Thus, the location estimation systemenables complicated analysis to be executed on the wave data byimplementing the location estimator using, for example, a sophisticatedcomputer. This allows the wave transmitted by the desired wavetransmission source to be more accurately identified. On the other hand,in the location estimation system, the wave receiver included in thewave reception node need only convert the received wave into wave datawithout analyzing the wave. Thus, the second embodiment can besimplified compared to the first embodiment. In addition, the locationestimation system allows effects identical or similar to the effects ofthe first embodiment to be exerted.

Third Embodiment

As illustrated in FIG. 9, a location estimation system according to athird embodiment includes four wave transmission and reception nodes600-1, 600-2, 600-3, and 600-4 and a location estimator 640. The totalnumber of wave transmission and reception nodes 600 may be five or more.Furthermore, some of the wave transmission and reception nodes 600 maybe replaced with wave reception nodes.

In the location estimation system in FIG. 9, one of the wavetransmission and reception nodes 600 (that is assumed to be, forexample, the wave transmission and reception node 600-1) transmits awave, and the remaining wave transmission and reception nodes 600 (thatare assumed to be, for example, the wave transmission and receptionnodes 600-2, 600-3, and 600-4) receive the waves. The location estimator640 can calculate the wave propagation times of propagation for therespective wave transmission and reception nodes 600-2, 600-3, and 600-4by subtracting the point in time of transmission of the wave by the wavetransmission and reception node 600-1 from the point in time ofreception of the wave by each of the wave transmission and receptionnodes 600-2, 600-3, and 600-4. Based on the wave propagation times, thelocation estimator 640 estimates the location of the wave transmissionand reception node 600 having transmitted the wave and estimates thedistance between the wave transmission and reception node 600 havingtransmitted the wave and any of the wave transmission and receptionnodes 600 having received the wave.

The wave transmission and reception node 600 can transmit a wave (forexample, a sound wave). The wave may have a predetermined characteristicin order to be distinguished from other waves. The wave transmission andreception node 600 may be an apparatus such as a robot with aloudspeaker. Upon transmitting the wave, the wave transmission andreception node 600 acquires information indicative of the correspondingtime (hereinafter referred to as transmission time information). Thetransmission time information may be, for example, a timer value. Inorder to reduce the magnitude of location estimation errors, timersreferenced by the wave transmission and reception nodes 600-1, 600-2,600-3, and 600-4 preferably synchronize accurately with one another.Moreover, the wave transmission and reception node 600 can receive thewave. Upon detecting that the received wave has the predeterminedcharacteristic, the wave transmission and reception node 600 acquiresreception time information.

Specifically, the wave transmission and reception node 600 includes awave receiver 110, a wireless communicator 620, and a wave transmitter660. The wave transmitter 660 transmits the wave with the predeterminedcharacteristic and notifies the wireless communicator 620 that the wavetransmitter 660 has transmitted the wave.

Upon receiving a notification from the wave receiver 110, the wirelesscommunicator 620 accesses a built-in timer to acquire the reception timeinformation. Upon receiving the notification from the wave transmitter660, the wireless communicator 620 accesses the built-in timer toacquire the transmission time information. The wireless communicator 620outputs the reception time information or the transmission timeinformation to the location estimator 640. The location estimator 640may be connected to any one of the wave transmission and reception nodes600-1, 600-2, 600-3, and 600-4 in a wired or wireless manner. Whendirectly connected to the location estimator 640, the wirelesscommunicator 620 outputs the reception time information or thetransmission time information directly to the location estimator 640. Onthe other hand, when not directly connected to the location estimator640, the wireless communicator 620 outputs the reception timeinformation or the transmission time information to the locationestimator 640 via another wave transmission and reception node 600.

A specific example of the wave transmission and reception node 600 isdepicted in FIG. 10. The wireless communicator 620 includes acommunication controller 621, a synchronization timer 122, a wirelessreceiver 623, and a wireless transmitter 624. The wave transmitter 660includes a loudspeaker 661 and a loudspeaker controller 662.

The loudspeaker controller 662 gives a transmission instruction to theloudspeaker 661 at a predetermined timing to allow the loudspeaker 661to transmit a sound wave with a predetermined characteristic. Theloudspeaker controller 662 may give the transmission instruction uponreceiving the transmission instruction from the communication controller621. Moreover, the loudspeaker controller 662 notifies the communicationcontroller 621 that the loudspeaker 661 has transmitted the sound wave.

Upon receiving the transmission instruction, the loudspeaker 661transmits the sound wave with the predetermined characteristic. Forexample, the sound wave transmitted by the loudspeaker 661 may have apredetermined sound pressure level, a predetermined time waveform, or apredetermined frequency component.

The communication controller 621 controls the synchronization timer 122,the wireless receiver 623, and the wireless transmitter 624. Forexample, upon receiving a notification from the detector 112 or theloudspeaker controller 662, the communication controller 621 acquiresthe timer value of the synchronization timer 122. The timer valuecorresponds to the reception time information or the transmission timeinformation. The communication controller 621 outputs the reception timeinformation or the transmission time information to the locationestimator 640. The communication controller 621 may further output, inaddition to the timer value, information indicating whether the timervalue corresponds to the reception time information or the transmissiontime information.

Specifically, when directly connected to the location estimator 640, thewireless communicator 621 outputs the reception time information and thetransmission time information directly to the location estimator 640. Onthe other hand, when not directly connected to the location estimator640, the wireless communicator 621 outputs the reception timeinformation or the transmission time information to the locationestimator 640 via another wave transmission and reception node 600. Whenthe wireless receiver 623 receives the reception time information or thetransmission time information from another wave transmission andreception node 600, the communication controller 621 similarly outputsthe reception time information or the transmission time information tothe location estimator 640.

Moreover, the communication controller 621 executes a synchronizationprocess for synchronizing the synchronization timer 122 with thesynchronization timer 122 built in another wave transmission andreception node 600. For example, the communication controller 621 mayallow the wireless transmitter 624 to transmit the timer value of thesynchronization timer 122 to another wave transmission and receptionnode 600 or may correct the timer value of the synchronization timer 122using the timer value in another wave transmission and reception node600 received by the wireless receiver 623.

In addition, the communication controller 621 may give a transmissioninstruction to the loudspeaker controller 662 to allow the loudspeakercontroller 662 to transmit a sound wave through the loudspeaker 661.Upon receiving a transmission instruction from the location estimator640, the communication controller 621 may give the transmissioninstruction to the loudspeaker controller 662. In this case, thelocation estimator 640 need not directly be connected to thecommunication controller 621. For example, the transmission instructionmay be wirelessly transmitted by another wireless communicator 620.

Specifically, the communication controller 621 may operate asillustrated in FIG. 11. First, the communication controller 621continues to wait for a notification from the detector 112 (notificationof reception) or a notification from the loudspeaker controller 662(notification of transmission) (step S301). Then, upon receiving thetransmission notification or the reception notification, thecommunication controller 621 acquires the timer value of thesynchronization timer 122 (step S302).

When directly connected to the location estimator 640, the communicationcontroller 621 outputs the timer value acquired in step S302 directly tothe location estimator 640 (step S303 and step S304). On the other hand,when not directly connected to the location estimator 640, thecommunication controller 621 allows the wireless transmitter 624 totransmit the timer value acquired in step S302 to the location estimator640 or another wave transmission and reception node 600 (step S303 andstep S305).

The wireless receiver 623 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 623receives the reception time information or the transmission timeinformation, and information needed for the synchronization process (forexample, the timer value of the synchronization timer 122 built inanother wave transmission and reception node 600) from another wavetransmission and reception node 600 in the form of wireless signals.

The wireless transmitter 624 transmits various types of information inthe form of wireless signals. Specifically, the wireless transmitter 624transmits the reception time information or the transmission timeinformation, and information needed for the synchronization process (forexample, the timer value of the synchronization timer 122) to anotherwave transmission and reception node 600, the location estimator 640,and the like in the form of wireless signals. The wireless transmitter624 may transmit the identifier of the wave transmission and receptionnode 600 together with the reception time information or thetransmission time information.

The location estimator 640 collects the reception time information andthe transmission time information from the wave transmission andreception nodes 600-1, 600-2, 600-3, and 600-4. The location estimator640 estimates the location of the wave transmission and reception node600 having transmitted the wave based on the collected reception timeinformation and transmission time information, the velocity of the wave,and the location of the wave transmission and reception node 600 havingreceived the wave. The location estimator 640 can reference, forexample, the identifier and location of the wave transmission andreception node 600 having received the wave, and the velocity of thewave. The identifier and location of the wave transmission and receptionnode 600 having received the wave may be registered when the wavetransmission and reception node 600 is installed.

When the location of the wave transmission and reception node 600-j (jis any integer of at least 1 and at most 4) having transmitted the waveis represented as (x, y, z), the location of the wave transmission andreception node 600-i (i is an integer which is at least 1 and at most 4and which is different from j) having received the wave is representedas (x_(i), y_(i), z_(i)), the time of wave propagation from the wavetransmission and reception node 600-j to the wave transmission andreception node 600-i is represented as t_(i), and the velocity of thewave (for example, a sound wave) is represented as v, then Expression(4) holds true.

$\begin{matrix}{{vt}_{i} = \sqrt{\left( {x - x_{i}} \right)^{2} + \left( {y - y_{i}} \right)^{2} + \left( {z - z_{i}} \right)^{2}}} & (4)\end{matrix}$

In Expression (4), v, x_(i), y_(i), and z₁ are known to the locationestimator 640. Moreover, t_(i) can be calculated using the differencebetween the reception time information collected from the wavetransmission and reception node 600-i and the transmission timeinformation collected from the wave transmission and reception node600-j. Hence, the location estimator 640 can calculate the location (x,y, z) of the wave transmission and reception node 600-j by solving alinear simultaneous equation with three unknowns derived from Expression(4). Moreover, the location estimator 140 can calculate the distancefrom the wave transmission and reception node 600-j to the wavetransmission and reception node 600-i by, for example, substitutingt_(i) into the left side of Expression (4).

When the total number of wave transmission and reception nodes 600 istwo, the location estimator 640 can calculate, by allowing one of thewave transmission and reception nodes 600 to transmit the wave, thedistance from this wave transmission and reception node 600 to the otherwave transmission and reception node 600 based on Expression (4)described above. Moreover, when a new wave transmission and receptionnode 600 is additionally installed, the location estimator 640 cancalculate, by allowing the new wave transmission and reception node 600to transmit the wave, the distance from the new wave transmission andreception node 600 to each of the two other wave transmission andreception nodes 600 based on Expression (4) described above.

Moreover, given that the locations of three or more wave transmissionand reception node 600 are known, the location estimator 640 canestimate the locations of the wave transmission and reception nodes 600with unknown locations by allowing the wave transmission and receptionnodes 600 with unknown locations to transmit the wave. Hence, regardlessof the total number of wave transmission and reception nodes 600, aslong as an installer or the like registers the locations of at leastthree wave transmission and reception nodes 600, the location estimator640 can estimate the locations of the remaining wave transmission andreception nodes 600. That is, regardless of the scale of the locationestimation system, the burden of a location measuring operation on theinstaller or the like can be reduced.

The wireless communicators 620-1, 620-2, 620-3, and 620-4 may form astar network instead of a peer-to-peer network. Furthermore, a masterstation may be provided independently of the wave transmission andreception node 600. Moreover, the location estimator 640 may beincorporated into any one of the wireless communicators 620.

As described above, the location estimation system according to thethird embodiment includes the four or more wave transmission and thereception nodes, and the wireless communicators included in therespective wave transmission and reception nodes form a wirelesscommunication network. The wave transmission and reception nodetransmitting the wave acquires transmission time information on the wavefrom the timer built in the wireless communicator in the wavetransmission and reception node and outputs the transmission timeinformation to the location estimator. On the other hand, the remainingwave transmission and reception nodes receiving the wave acquirereception time information on the wave from the timers built in thewireless communicators in the wave transmission and reception nodes andoutput the reception time information to the location estimator.

The location estimator estimates the location of the wave transmissionand reception node having transmitted the wave based on the collectedtransmission time information and reception time information, thevelocity of the wave, and the locations of the wave transmission andreception nodes having received the wave. The timers synchronizeaccurately with one another among the wave transmission and receptionnodes, and thus, the location estimator can collect accurate wavepropagation times for the wave transmission and reception nodes havingreceived the wave and accurately estimate the location of the wavetransmission and reception node having transmitted the wave.

Furthermore, in the location estimation system, when a new wavetransmission and reception node is additionally installed, the locationof the new wave transmission and reception node can be estimated byallowing the new wave transmission and reception node to transmit thewave. Thus, the installer or the like need not actually measure theadditionally installed wave transmission and reception node.

Moreover, the wave transmission and reception node can acquire thetransmission time information and the reception time information fromthe timer built in the wireless communicator in the wave transmissionand reception node. That is, the wave transmission and reception nodeneed not separately have, for example, hardware or software configuredto execute a time synchronization process based on the NTP. The locationestimation system needs no special apparatus for time synchronizationsuch as an NTP server. In addition, the wave reception nodes arewirelessly connected together. Thus, no communication cable needs to belaid, and the wireless communicator included in the wave transmissionand reception node may be implemented using, for example,general-purpose wireless LAN equipment compliant with IEEE 802.11.Hence, the location estimation system can be inexpensively, simply, andcompactly constructed, and the apparatuses included in the locationestimation system can be easily installed.

Fourth Embodiment

As illustrated in FIG. 12, a location estimation system according to afourth embodiment includes four wave transmission nodes 700-1, 700-2,700-3, and 700-4, a location estimator 740, and a wave reception node770. The total number of wave transmission nodes 700 may be five ormore. Furthermore, the wave transmission node 700 or the wave receptionnode 770 may be replaced with a wave transmission and reception node.

The wave transmission node 700 can transmit a wave (for example, a soundwave). The wave may have a predetermined characteristic in order to bedistinguished from other waves. The wave transmission node 700 may be anapparatus such as a robot with a loudspeaker.

Specifically, the wave transmission node 700 includes a wirelesscommunicator 720 and a wave transmitter 760. The wave transmitter 760transmits the wave with the predetermined characteristic. A timing whenthe wave transmitter 760 transmits the wave is controlled by thewireless communicator 720.

Each wireless communicator 720 references a timer synchronized with atimer built in another wireless communicator 720 to control the timingwhen the wave transmitter 760 is allowed to transmit the wave based onthe timer value of the timer.

A specific example of the wave transmission node 700 is depicted in FIG.13. The wireless communicator 720 includes a communication controller721, a synchronization timer 122, a wireless receiver 723, and awireless transmitter 724. The wave transmitter 760 includes aloudspeaker 661 and a loudspeaker controller 762.

Upon receiving a transmission instruction from the communicationcontroller 721, the loudspeaker controller 762 gives a transmissioninstruction to the loudspeaker 661 to allow the loudspeaker 661 totransmit the sound wave with the predetermined characteristic. Thepredetermined characteristic is explicitly or implicitly indicative ofthe identifier of the wave transmission node 700. For example, for thesound wave transmitted from the loudspeaker 661, at least one of theamplitude, phase, and frequency thereof may be modulated based on theidentifier of the wave transmission source node 700.

The communication controller 721 controls the synchronization timer 122,the wireless receiver 723, the wireless transmitter 724, and theloudspeaker controller 762. For example, the communication controller721 executes a synchronization process for synchronizing thesynchronization timer 122 with the synchronization timer 122 built inanother wave transmission node 700. For example, the communicationcontroller 721 may allow the wireless transmitter 724 to transmit thetimer value of the synchronization timer 122 to another wavetransmission node 700 or may correct the timer value of thesynchronization timer 122 using the timer value in another wavetransmission node 700 received by the wireless receiver 723.

In addition, the communication controller 721 may give a transmissioninstruction to the loudspeaker controller 762 to allow the loudspeakercontroller 762 to transmit a sound wave through the loudspeaker 661 at apredetermined timing. For example, the communication controller 721 mayreference the timer value of the synchronization timer 122 and give atransmission instruction to the loudspeaker controller 762 when thetimer value reaches a predetermined value corresponding to thepredetermined timing.

Alternatively, the communication controller 721 may output a pulse wavewith a predetermined period to the loudspeaker controller 762 based onthe timer value of the synchronization timer 122. In this case, theloudspeaker controller 762 allows the loudspeaker 661 to transmit thesound wave in accordance with the pulse wave received from thecommunication controller 721. Furthermore, the frequency and phase ofthe pulse wave may be preset to predetermined values via wirelesscommunication among wireless communicators 720-1, 720-2, 720-3, and720-4. In this case, the communication controllers 721-1, 721-2, 721-3,and 721-4 can output a pulse wave with a synchronized frequency and asynchronized phase. That is, loudspeakers 661-1, 661-2, 661-3, and 661-4can periodically and concurrently transmit the respective sound waves.

The wireless receiver 723 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 723receives information needed for the synchronization process (forexample, the timer value of the synchronization timer 122 built inanother wave reception node 700) from another wave reception node 100 inthe form of wireless signals.

The wireless transmitter 724 transmits various types of information inthe form of wireless signals. Specifically, the wireless transmitter 724transmits information needed for the synchronization process (forexample, the timer value of the synchronization timer 122) to anotherwave reception node 700 in the form of wireless signals.

The wave reception node 770 receives the wave. Upon detecting that thereceived wave has the above-described characteristic, the wave receptionnode 770 notifies the location estimator 740 that the wave receptionnode 770 has received the wave. The location estimator 740 is depictedto be incorporated in the wave reception node 770 in FIG. and FIG. 14,but may be provided outside the wave reception node 770.

A specific example of the wave reception node 770 is depicted in FIG.14. The wave reception node 770 includes a wave receiver 710 and alocation estimator 740. The wave receiver 710 receives the wavestransmitted by the wave transmission nodes 700. Upon detecting that thereceived wave has the above-described characteristic, the wave receiver710 notifies the location estimator 740 that the wave receiver 710 hasreceived the desired wave in addition to notifying the locationestimator 740 of the identifier of the wave transmission node 700 havingtransmitted the wave.

Upon receiving the notification from the wave receiver 710, the locationestimator 740 accesses a built-in timer to acquire the reception timeinformation. The timer may be built into the wave receiver 710 (forexample, the detector 712) instead of the location estimator 740. Inthis case, the location estimator 740 can acquire the reception timeinformation from the wave receiver 710. The timer may be referenced inorder to calculate wave propagation time differences and need notsynchronize with the synchronization timer 122. The location estimator740 thus collects the reception time information on the wave transmittedby each wave transmission node 700 from the wave reception node 770.

The location estimator 740 estimates the location of the wave receptionnode 770 based on the collected reception time information, the velocityof the wave, and the location of each wave transmission node 700. Thelocation estimator 740 can reference, for example, the identifier andlocation of each wave transmission node 700 and the velocity of thewave. The identifier and location of each wave transmission node 700 maybe registered when the wave transmission node 700 is installed.

When the location of the wave reception node 770 is represented as (x,y, z), the location of the wave transmission node 700-i (i is an integerof at least 1 and at most 4) is represented as (x_(i), y_(i), z_(i)),the time of wave propagation from the wave transmission node 700-i tothe wave reception node 770 is represented as t_(i), and the velocity ofthe wave (for example, a sound wave) is represented as v, thenExpression (1), described above, holds true.

Moreover, it is assumed that the wave transmission nodes 700-1, 700-2,700-3, and 700-4 have transmitted the wave at the same timing. Then,when, for i >1, the difference between the reception time information onthe wave from the wave transmission node 700-i and the reception timeinformation on the wave from the wave transmission node 700-1 isrepresented as Δt_(i), Expression (2), described above, holds true. Thetimings when the wave transmission nodes 700-1, 700-2, 700-3, and 700-4need not be the same, and a predetermined time difference may be presentamong the timings. However, in particular, when the wave reception node770 moves at high speed, the waves are preferably transmitted at thesame timing in order to suppress location estimation errors.

Based on Expression (1) and Expression (2), Expression (3), describedabove, holds true for unknowns x, y, z, and t₁. In Expression (3), v,x₁, y₁ z₁, x₂, y₂, z₂, x₃, y₃, z₃, x₄, y₄, and z₄ are known to thelocation estimator 740. Furthermore, the location estimator 740 cancalculate Δt₂, Δt₃, and Δt₄ based on the collected reception timeinformation. Hence, the location estimator 740 can calculate thelocation (x, y, z) of the wave reception node 770 by solving Expression(3). Moreover, the location estimator 740 can calculate the distancefrom the wave transmission node 700-i to the wave reception node 770 by,for example, substituting x, y, and z into the right side of Expression(1).

The location estimation system in FIG. 12 operates as illustrated inFIG. 17.

In each wave transmission node 700, the wireless communicator 720 allowsthe wave transmitter 760 to transmit a sound wave. The sound wavetransmitted by each wave transmitter 760 is received by the wavereceiver 710 in a propagation time commensurate with the distance fromthe wave transmitter 760 to the wave receiver 710.

Upon detecting that the received sound wave has a predeterminedcharacteristic corresponding to the identifier of one of the wavetransmission nodes 700, the wave receiver 710 notifies the locationestimator 740 that the wave receiver 710 has received the identifier andthe sound wave.

Upon receiving, from the wave receiver 710, the first notification thatthe wave receiver 710 has received the sound wave, the locationestimator 740 starts measuring the propagation time differences of theother waves with reference to the reception time information on thesound wave received by the wave receiver 710. For example, the locationestimator 740 may start the built-in timer. Subsequently, each time thelocation estimator 740 receives, from the wave receiver 710, anotification that the wave receiver 710 has received the sound wave, thelocation estimator 740 measures the propagation time differences of thesound waves. The propagation time differences correspond to Δt₂, Δt₃,Δt₄ in Expression (3), described above. The location estimator 740 canestimate the location of the wave reception node 770 provided that thelocation estimator 740 receives sound waves from at least four wavetransmission nodes 700. Hence, given that the number of sound wavesreceived has reached four, the location estimator 740 can end themeasurement of the wave propagation time differences. For example, thelocation estimator 740 may stop the built-in timer. The locationestimator 740 then estimates the location of the wave reception node 770based on the measured propagation time differences, the velocity of thewave, and the location of each wave transmission node 700.

The wireless communicators 720-1, 720-2, 720-3, and 720-4 may form astar network instead of a peer-to-peer network. Furthermore, a masterstation may be provided independently of the wave transmission node 700.

As described above, the location estimation system according to thefourth embodiment includes the four or more wave transmission nodes, andthe wireless communicators included in the respective wave transmissionnodes form a wireless communication network. The location estimatorcollects the reception time information on the wave transmitted by eachwave transmission node from the wave reception node.

The location estimator estimates the location of the wave reception nodebased on the collected reception time information, the velocity of thewave, and the location of each wave transmission node. The timerssynchronize accurately with one another among the wave transmissionnodes, and thus, the location estimator can collect accurate wavepropagation time differences among the wave transmission nodes andaccurately estimate the location of the wave reception node.

Furthermore, the wave transmission node can synchronously transmit thesound wave based on the timer value of the timer built in the wirelesscommunicator in the wave transmission node. That is, the wavetransmission node need not separately have, for example, hardware orsoftware configured to execute a time synchronization process based onthe NTP. The location estimation system needs no special apparatus fortime synchronization such as an NTP server. In addition, the wavetransmission nodes are wirelessly connected together. Thus, nocommunication cable needs to be laid, and the wireless communicatorincluded in the wave transmission node may be implemented using, forexample, general-purpose wireless LAN equipment compliant with IEEE802.11. Hence, the location estimation system can be inexpensively,simply, and compactly constructed, and the apparatuses included in thelocation estimation system can be easily installed.

Fifth Embodiment

As illustrated in FIG. 15, a location estimation system according to afifth embodiment includes three wave transmission nodes 800-1, 800-2,and 800-3, a location estimator 840, and a wave reception node 870. Thetotal number of wave transmission nodes 800 may be four or more.Furthermore, the wave transmission node 800 or the wave reception node870 may be replaced with a wave transmission and reception node.

In the location estimation system in FIG. 15, the wave transmissionnodes 800-1, 800-2, and 800-3 transmit waves, and the wave receptionnodes 870 receives the waves. The location estimator 840 can calculatethe wave propagation times from the respective wave transmission nodes800-1, 800-2, and 800-3 by subtracting the point in time of transmissionof the wave by each of the wave transmission nodes 800-1, 800-2, and800-3 from the point in time of reception of the wave by the wavereception node 870. Based on the wave propagation times, the locationestimator 840 estimates the location of the wave reception node 870 andestimates the distance between the wave reception node 870 and any ofthe wave transmission nodes 800-1, 800-2, and 800-3.

The wave transmission node 800 can transmit a wave (for example, a soundwave). The wave may have a predetermined characteristic in order to bedistinguished from other waves. The wave transmission node 800 may be anapparatus such as a robot with a loudspeaker. Upon transmitting thewave, the wave transmission node 800 acquires the transmission timeinformation.

Specifically, the wave transmission node 800 includes a wirelesscommunicator 820 and a wave transmitter 760. The wave transmitter 760may transmit the wave with the predetermined characteristic and notifythe wireless communicator 820 that the wave transmitter 760 hastransmitted the wave. The timing when the wave transmitter 760 transmitsthe wave is controlled by the wireless communicator 820.

Upon receiving a notification from the wave transmitter 760, thewireless communicator 820 accesses a built-in timer to acquire thetransmission time information. Alternatively, the wireless communicator820 may access the built-in timer to acquire the transmission timeinformation when giving the wave transmitter 760 a transmissioninstruction to transmit the wave. In this case, the wave transmitter 760needs no notification function.

The wireless communicator 820 outputs the transmission time informationto the location estimator 840. The location estimator 840 may beconnected to any one of the wave transmission nodes 800-1, 800-2, and800-3 in a wired or wireless manner. When directly connected to thelocation estimator 840, the wireless communicator 820 outputs thetransmission time information directly to the location estimator 840. Onthe other hand, when not directly connected to the location estimator840, the wireless communicator 820 outputs the transmission timeinformation to the location estimator 840 via another wave transmissionnode 800. The wave transmission nodes 800-1, 800-2, and 800-3synchronously transmit the waves, and thus, have substantially the sametransmission time information. Therefore, when not directly connected tothe location estimator 840, the wireless communicator 820 may omitoutputting the transmission time information.

Moreover, the wireless communicator 820 references a timer synchronizedwith a timer built in another wireless communicator 820, and based onthe timer value of the timer, controls the timing when the wavetransmitter 760 is allowed to transmit the wave.

Specifically, the wireless communicator 820 includes a communicationcontroller 821, a synchronization timer 122, a wireless receiver 823,and a wireless transmitter 824.

The communication controller 821 controls the synchronization timer 122,the wireless receiver 823, and the wireless transmitter 824. Forexample, upon receiving a notification from the loudspeaker controller762, the communication controller 821 acquires the timer value of thesynchronization timer 122. The timer value corresponds to thetransmission time information. The communication controller 821 outputsthe transmission time information to the location estimator 840.

Specifically, when directly connected to the location estimator 840, thewireless communicator 821 outputs the transmission time informationdirectly to the location estimator 840. On the other hand, when notdirectly connected to the location estimator 840, the wirelesscommunicator 821 allows the wireless transmitter 824 to transmit thetransmission time information to the location estimator 840 or anotherwave transmission node 800. When the wireless receiver 823 receives thetransmission time information from the wave transmission node 800, thecommunication controller 821 similarly outputs the transmission timeinformation to the location estimator 840.

Moreover, the communication controller 821 executes a synchronizationprocess for synchronizing the synchronization timer 122 with thesynchronization timer 122 built in another wave transmission node 800and the wave reception node 870. For example, the communicationcontroller 821 may allow the wireless transmitter 824 to transmit thetimer value of the synchronization timer 122 to another wavetransmission node 800 and the wave reception node 870 or may correct thetimer value of the synchronization timer 122 using the timer value inanother wave transmission node 800 or the timer value in the wavereception node 870, received by the wireless receiver 823.

In addition, the communication controller 821 gives a transmissioninstruction to the loudspeaker controller 762 to allow the loudspeakercontroller 762 to transmit a sound wave through the loudspeaker 661 at apredetermined timing. For example, the communication controller 821 mayreference the timer value of the synchronization timer 122 and give thetransmission instruction to the loudspeaker controller 762 when thetimer value reaches a predetermined value corresponding to thepredetermined timing.

Alternatively, the communication controller 821 may output a pulse wavewith a predetermined period to the loudspeaker controller 762 based onthe timer value of the synchronization timer 122. In this case, theloudspeaker controller 762 allows the loudspeaker 661 to transmit thesound wave in accordance with the pulse wave received from thecommunication controller 821. Furthermore, the frequency and phase ofthe pulse wave may be preset to predetermined values via wirelesscommunication among wireless communicators 720-1, 720-2, and 720-3. Inthis case, the communication controllers 821-1, 821-2, and 821-3 canoutput a pulse wave with a synchronized frequency and a synchronizedphase. That is, loudspeakers 661-1, 661-2, and 661-3 can periodicallyand concurrently transmit the respective sound waves.

The wireless receiver 823 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 823receives information needed for the synchronization process thereception time information or the transmission time information, and(for example, the timer values of the synchronization timer 122 built inanother wave transmission node 800 or the synchronization timer 122built in the wave reception node 870) from another wave transmissionnode 800 or the wave reception node 800 in the form of wireless signals.

The wireless transmitter 824 transmits various types of information inthe form of wireless signals.

Specifically, the wireless transmitter 824 transmits the transmissiontime information and information needed for the synchronization process(for example, the timer value of the synchronization timer 122) toanother wave transmission node 800 and the location estimator 840 in theform of wireless signals. The wireless transmitter 824 may transmit theidentifier of the wave transmission node 800 along with the transmissiontime information.

The wave reception node 870 receives the wave. Upon detecting that thereceived wave has the above-described characteristic, the wave receptionnode 870 acquires the reception time information. Specifically, the wavereception node 870 includes a wave receiver 710 and a wirelesscommunicator 880.

Each time the wireless communicator 880 receives a notification from thewave receiver 710, the wireless communicator 880 accesses a built-intimer to acquire the reception time information. The wirelesscommunicator 880 outputs the reception time information and theidentifier of the corresponding wave transmission node 800 to thelocation estimator 840. The location estimator 840 may be connected toany one of the wave transmission nodes 800-1, 800-2, and 800-3 in awired or wireless manner. In this case, the wireless communicator 880outputs the reception time information and the identifier to thelocation estimator 840 via the wave transmission node 800.

A specific example of the wave reception node 870 is depicted in FIG.16. The wireless communicator 880 includes a communication controller881, the synchronization timer 122, a wireless receiver 883, and awireless transmitter 884.

The communication controller 881 controls the synchronization timer 122,the wireless receiver 883, and the wireless transmitter 884. Forexample, upon receiving a notification from the detector 712 (thenotification includes the identifier of the wave transmission node 800corresponding to the transmission source for the wave received by thewave receiver 710), the communication controller 881 acquires the timervalue of the synchronization timer 122. The timer value corresponds tothe reception time information. The communication controller 881 outputsthe reception time information and the identifier of the correspondingwave transmission node 800 to the location estimator 840. Specifically,the communication controller 881 allows the wireless transmitter 884 totransmit the reception time information and the identifier to thelocation estimator 840 or the wave transmission nodes 800.

Moreover, the communication controller 881 executes a synchronizationprocess for synchronizing the synchronization timer 122 with thesynchronization timer 122 built in the wave transmission node 800. Forexample, the communication controller 881 may allow the wirelesstransmitter 884 to transmit the timer value of the synchronization timer122 to the wave transmission node 800 and the wave reception node 870 ormay correct the timer value of the synchronization timer 122 using thetimer value in the wave transmission node 800 received by the wirelessreceiver 883.

The wireless receiver 883 receives various types of information in theform of wireless signals. Specifically, the wireless receiver 883receives information needed for the synchronization process (forexample, the timer value of the synchronization timer 122 built in thewave transmission node 800) from the wave transmission node 800 in theform of wireless signals.

The wireless transmitter 884 transmits various types of information inthe form of wireless signals. Specifically, the wireless transmitter 884transmits the reception time information and the identifier of thecorresponding wave transmission node 800, and information needed for thesynchronization process (for example, the timer value of thesynchronization timer 122) to the wave transmission nodes 800, thelocation estimator 840, and the like in the form of wireless signals.

The location estimator 840 collects the transmission time informationfrom the wave transmission nodes 800-1, 800-2, and 800-3 and collects,from the wave reception node 870, the reception time information on thewave transmitted by each wave transmission node 800. The locationestimator 840 estimates the location of the wave reception node 870based on the collected reception time information and transmission timeinformation, the velocity of the wave, and the location of the wavetransmission node 800. The location estimator 840 can reference, forexample, the identifier and location of the wave transmission node 800and the velocity of the wave. The identifier and location of the wavetransmission node 800 may be registered when the wave transmission node800 is installed.

When the location of the wave reception node 870 is represented as (x,y, z), the location of the wave transmission node 800-i (i is an integerof at least 1 and at most 3) is represented as (x_(i), y_(i), z_(i)),the time of wave propagation from the wave transmission node 800-i tothe wave reception node 870 is represented as t_(i), and the velocity ofthe wave (for example, a sound wave) is represented as v, thenExpression (4), described above, holds true.

In Expression (4), v, x_(i), y_(i), and z₁ are known to the locationestimator 840. Moreover, t_(i) can be calculated using the differencebetween the transmission time information collected from the wavetransmission node 800-i and the reception time information collectedfrom the wave reception node 870 and corresponding to the wavetransmission node 800-j. Hence, the location estimator 840 can calculatethe location (x, y, z) of the wave reception node 870 by solving alinear simultaneous equation with three unknowns derived from Expression(4). Moreover, the location estimator 840 can calculate the distancefrom the wave reception node 870 to the wave transmission and receptionnode 800-i by, for example, substituting t_(i) into the left side ofExpression (4).

Even when the wave transmission node 800 does not transmit thetransmission time information, the location estimator 840 can estimatethe location of the wave reception node by solving Expression (3),described above, as long as the total number of wave transmission nodes800 is four or more.

The location estimation system in FIG. 15 operates as illustrated inFIG. 18.

In each wave transmission node 800, the wireless communicator 820 allowsthe wave transmitter 760 to transmit a sound wave. The sound wavetransmitted by each wave transmitter 760 is received by the wavereceiver 710 in a propagation time commensurate with the distance fromthe wave transmitter 760 to the wave receiver 710.

The wireless communicator 820 accesses the built-in synchronizationtimer 122 to acquire the transmission time information when giving thewave transmitter 760 a transmission instruction to transmit the wave.The wireless communicator 820 then outputs the transmission timeinformation to the location estimator 840.

Upon detecting that the received sound wave has a predeterminedcharacteristic corresponding to the identifier of one of the wavetransmission nodes 800, the wave receiver 710 notifies the wirelesscommunicator 880 that the wave receiver 710 has received the identifierand the sound wave.

Each time the wireless communicator 880 receives, from the wave receiver710, a notification that the wave receiver 710 has received the soundwave, the wireless communicator 880 acquires the reception timeinformation on the sound wave from the synchronization timer 122. Thelocation estimator 840 can estimate the location of the wave receptionnode 870 provided that the wireless communicator 880 receives the soundwave from at least three wave transmission nodes 800. Hence, given thatthe number of sound waves received has reached three, the wirelesscommunicator 880 transmits at least three sets of reception timeinformation (timer values) and the identifiers of the corresponding wavetransmission nodes 800 to the location estimator 840.

The location estimator 840 estimates the location of the wave receptionnode 870 based on the transmission time information and the receptiontime information, the velocity of the wave, and the location of eachwave transmission node 800.

The wireless communicators 820-1, 820-2, 820-3, and 880 may form a starnetwork instead of a peer-to-peer network. Furthermore, a master stationmay be provided independently of the wave transmission node 800 and thewave reception node 870. Moreover, the location estimator 840 may beincorporated into any of the wireless communicators 820-1, 820-2, and820-3.

As described above, the location estimation system according to thefifth embodiment includes the three or more wave transmission nodes andthe wave reception node, and the wireless communicators included in therespective wave transmission nodes and in the wave reception node form awireless communication network. At least one wave transmission nodeacquires the transmission time information from the timer built in thewireless communicator in the wave transmission node and outputs thetransmission time information to the location estimator. On the otherhand, the wave reception node acquires the reception time information onthe wave transmitted by each wave transmission node, from the timerbuilt in the wave reception node, and outputs the reception timeinformation to the location estimator along with the identifier of thewave transmission node.

The location estimator estimates the location of the wave reception nodebased on the collected transmission time information and reception timeinformation, the velocity of the wave, and the location of each wavetransmission node. The timers synchronize accurately with one anotheramong the wave transmission nodes and the wave reception node, and thus,the location estimator can collect accurate wave propagation times fromthe wave transmission nodes and accurately estimate the location of thewave reception node.

Furthermore, the wave transmission node can synchronously transmit thesound wave based on the timer value of the timer built in the wirelesscommunicator in the wave transmission node. Moreover, the wave receptionnode can acquire the reception time information from the timer built inthe wireless communicator in the wave reception node. That is, the wavetransmission node and the wave reception node need not separately have,for example, hardware or software configured to execute a timesynchronization process based on the NTP. The location estimation systemneeds no special apparatus for time synchronization such as an NTPserver. In addition, the wave transmission node and the wave receptionnode are wirelessly connected together. Thus, no communication cableneeds to be laid, and the wireless communicators included in the wavetransmission node and wave reception node may be implemented using, forexample, general-purpose wireless LAN equipment compliant with IEEE802.11. Hence, the location estimation system can be inexpensively,simply, and compactly constructed, and the apparatuses included in thelocation estimation system can be easily installed.

At least a part of the processing in the above-described embodiments canbe implemented using a general-purpose computer as basic hardware. Aprogram implementing the processing in each of the above-describedembodiments may be stored in a computer readable storage medium forprovision. The program is stored in the storage medium as a file in aninstallable or executable format. The storage medium is a magnetic disk,an optical disc (CD-ROM, CD-R, DVD, or the like), a magnetooptic disc(MO or the like), a semiconductor memory, or the like. That is, thestorage medium may be in any format provided that a program can bestored in the storage medium and that a computer can read the programfrom the storage medium. Furthermore, the program implementing theprocessing in each of the above-described embodiments may be stored on acomputer (server) connected to a network such as the Internet so as tobe downloaded into a computer (client) via the network.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. An estimation system comprising: a node groupthat includes a first node receiving a first wave and a second nodecommunicating wirelessly with the first node; and an estimator thatestimates, based on a propagation time of the first wave, (i) a distancefrom a first transmission source having transmitted the first wave tothe first node or (ii) a location of the first transmission source orthe first node, wherein the first node comprises: a first wave receiverthat receives the first wave; and a first wireless communicator thatincorporates a first timer, synchronizes the first timer with a secondtimer built in the second node via wireless communication with thesecond node, acquires a first piece of reception time informationindicative of a point in time of reception of the first wave from thefirst timer, and outputs the first piece of reception time informationto the estimator.
 2. The system according to claim 1, wherein the nodegroup further includes a third node and a fourth node that communicatewirelessly with the first node and the second node, the first wirelesscommunicator synchronizes the first timer with a second timer, a thirdtimer, and a fourth timer built in the second node, the third node, andthe fourth node via wireless communication with the second node, thethird node, and the fourth node, the second node comprises: a secondwave receiver that receives the first wave; and a second wirelesscommunicator that incorporates the second timer, synchronizes the secondtimer with the first timer, the third timer, and the fourth timer viawireless communication with the first node, the third node, and thefourth node, acquires a second piece of reception time informationindicative of the point in time of reception of the first wave from thesecond timer, and outputs the second piece of reception time informationto the estimator, the third node comprises: a third wave receiver thatreceives the first wave; and a third wireless communicator thatincorporates the third timer, synchronizes the third timer with thefirst timer, the second timer, and the fourth timer via wirelesscommunication with the first node, the second node, and the fourth node,acquires a third piece of reception time information indicative of thepoint in time of reception of the first wave from the third timer, andoutputs the third piece of reception time information to the estimator,the fourth node comprises: a fourth wave receiver that receives thefirst wave; and a fourth wireless communicator that incorporates thefourth timer, synchronizes the fourth timer with the first timer, thesecond timer, and the third timer via wireless communication with thefirst node, the second node, and the third node, acquires a fourth pieceof reception time information indicative of the point in time ofreception of the first wave from the fourth timer, and outputs thefourth piece of reception time information to the estimator, and theestimator calculates propagation time differences between a propagationtime of the first wave from the first transmission source to the firstnode and propagation times of the first wave from the first transmissionsource to the second node, the third node, and the fourth node based onthe first piece of reception time information, the second piece ofreception time information, the third piece of reception timeinformation, and the fourth piece of reception time information toestimate a location of the first transmission source based on thepropagation time differences.
 3. The system according to claim 1,wherein the first wave is a sound wave, the first wave receivercomprises: a microphone that receives and converts the sound wave intoan electric signal; and a detector that checks whether or not theelectric signal comprises a predetermined characteristic, and upondetecting that the electric signal comprises the predeterminedcharacteristic, notifies the first wireless communicator that a desiredsound wave has been received, and the first wireless communicator, uponreceiving the notification from the detector, acquires the first pieceof reception time information from the first timer.
 4. The systemaccording to claim 1, wherein the first wave is a sound wave, the firstwave receiver comprises: a microphone that receives and converts thesound wave into an electric signal; and a converter that converts theelectric signal into digital data, the first wireless communicatoroutputs the first piece of reception time information and the digitaldata to the estimator, and the estimator checks whether or not thedigital data comprises a predetermined characteristic, and upondetecting that the digital data comprises the predeterminedcharacteristic, estimates, based on the first piece of reception timeinformation corresponding to the digital data, (i) a distance from thefirst transmission source to the first node or (ii) a location of thefirst transmission source or the first node.
 5. The system according toclaim 1, wherein the first transmission source is the second node, thesecond node comprises: a first wave transmitter that transmits the firstwave; and a second wireless communicator that incorporates the secondtimer, synchronizes the second timer with the first timer via wirelesscommunication with the first node, acquires transmission timeinformation indicative of a point in time of transmission of the firstwave from the second timer, and outputs the transmission timeinformation to the estimator, and the estimator calculates a propagationtime of the first wave from the second node to the first node bysubtracting the transmission time information from the first piece ofreception time information, and based on the propagation time, estimatesa distance from the second node to the first node.
 6. The systemaccording to claim 5, wherein the second wireless communicator, uponwirelessly receiving an instruction to transmit the first wave, givesthe first wave transmitter the instruction to transmit the first wave,and the first wave transmitter, when given the instruction to transmitthe first wave by the second wireless communicator, transmits the firstwave.
 7. The system according to claim 1, wherein the node group furtherincludes a third node and a fourth node that communicate wirelessly withthe first node and the second node, the first transmission source is thefourth node, the first wireless communicator synchronizes the firsttimer with the second timer, a third timer, and a fourth timer built inthe second node, the third node, and the fourth node via wirelesscommunication with the second node, the third node, and the fourth node,the second node comprises: a second wave receiver that receives thefirst wave; and a second wireless communicator that incorporates thesecond timer, synchronizes the second timer with the first timer, thethird timer, and the fourth timer via wireless communication with thefirst node, the third node, and the fourth node, acquires a second pieceof reception time information indicative of the point in time ofreception of the first wave from the second timer, and outputs thesecond piece of reception time information to the estimator, the thirdnode comprises: a third wave receiver that receives the first wave; anda third wireless communicator that incorporates the third timer,synchronizes the third timer with the first timer, the second timer, andthe fourth timer via wireless communication with the first node, thesecond node, and the fourth node, acquires a third piece of receptiontime information indicative of the point in time of reception of thefirst wave from the third timer, and outputs the third piece ofreception time information to the estimator, the fourth node comprises:a first wave transmitter that transmits the first wave; and a fourthwireless communicator that incorporates the fourth timer, synchronizesthe fourth timer with the first timer, the second timer, and the thirdtimer via wireless communication with the first node, the second node,and the third node, acquires transmission time information indicative ofthe point in time of transmission of the first wave from the fourthtimer, and outputs the transmission time information to the estimator,and the estimator subtracts the transmission time information from thefirst piece of reception time information, the second piece of receptiontime information, and the third piece of reception time information tocalculate propagation times from the fourth node to the first node, thesecond node, and the third node, and based on the propagation times,estimates a location of the fourth node.
 8. The system according toclaim 7, wherein the fourth wireless communicator, upon wirelesslyreceiving an instruction to transmit the first wave, gives the firstwave transmitter the instruction to transmit the first wave, and thefirst wave transmitter, when given the instruction to transmit the firstwave by the fourth wireless communicator, transmits the first wave. 9.The system according to claim 1, wherein the node group further includesa third node and a fourth node that communicate wirelessly with thefirst node and the second node, the first transmission source is thesecond node, the second node comprises: a first wave transmitter thattransmits the first wave; and a second wireless communicator thatincorporates the second timer, synchronizes the second timer with thefirst timer, a third timer, and a fourth timer built in the first node,the third node, and the fourth node via wireless communication with thefirst node, the third node, and the fourth node, controls a timing whenthe first wave transmitter transmits the first wave based on a timervalue of the second timer, acquires transmission time informationindicative of a point in time of transmission of the first wave from thesecond timer, and outputs the transmission time information to theestimator, the third node comprises: a second wave transmitter thattransmits a second wave; and a third wireless communicator thatincorporates the third timer, synchronizes the third timer with thefirst timer, the second timer, and the fourth timer via wirelesscommunication with the first node, the second node, and the fourth node,and controls a timing when the second wave transmitter transmits thesecond wave based on a timer value of the third timer, the fourth nodecomprises: a third wave transmitter that transmits a third wave; and afourth wireless communicator that incorporates the fourth timer,synchronizes the fourth timer with the first timer, the second timer,and the third timer via wireless communication with the first node, thesecond node, and the third node, and controls a timing when the thirdwave transmitter transmits the third wave based on a timer value of thefourth timer, the first wave receiver receives the first wave, thesecond wave, and the third wave, the first wireless communicatorsynchronizes the first timer with the second timer, the third timer, andthe fourth timer via wireless communication with the second node, thethird node, and the fourth node, acquires a first piece of receptiontime information, a second piece of reception time information, and athird piece of reception time information indicative of the points intime of reception of the first wave, the second wave, and the thirdwave, respectively, from the first timer, and outputs the first piece ofreception time information, the second piece of reception timeinformation, and the third piece of reception time information to theestimator, and the estimator subtracts the transmission time informationfrom the first piece of reception time information, the second piece ofreception time information, and the third piece of reception timeinformation to calculate propagation times of the first wave, the secondwave, and the third wave from the second node, the third node, and thefourth node, respectively, to the first node, and based on thepropagation times, estimates a location of the first node.
 10. Thesystem according to claim 1, wherein a second wireless communicatorbuilt in the second node corresponds to a master station in a starnetwork and wirelessly transmits a timer value of the second timer tothe first wireless communicator, the first wireless communicatorcorresponds to a slave station in the star network, corrects the firsttimer based on the timer value of the second timer, and transmits thefirst piece of reception time information to the second wirelesscommunicator, and the second wireless communicator outputs the firstpiece of reception time information to the estimator.
 11. The systemaccording to claim 1, further comprising a wireless communicationapparatus that corresponds to a master station in a star network, andwherein the first wireless communicator and a second wirelesscommunicators built in the second node correspond to slave stations inthe star network, the first wireless communicator and the secondwireless communicator communicate wirelessly via the wirelesscommunication apparatus.
 12. The system according to claim 1, whereinthe estimator is wirelessly connected to a second wireless communicatorbuilt in the second node.
 13. The system according to claim 1, whereinthe estimator is built in a second wireless communicator built in thesecond node.
 14. An estimation system comprising: a transmission nodegroup that includes a first transmission node, a second transmissionnode, a third transmission node, and a fourth transmission node thattransmit a first wave, a second wave, a third wave, and a fourth waveand communicates wirelessly with one another; a reception node thatreceives the first wave, the second wave, the third wave, and the fourthwave; an estimator that estimates a location of the reception node basedon propagation times of the first wave, the second wave, the third wave,and the fourth wave, wherein the first transmission node comprises: afirst wave transmitter that transmits the first wave; and a firstwireless communicator that incorporates a first timer, synchronizes thefirst timer with a second timer, a third timer, and a fourth timer builtin the second transmission node, the third transmission node, and thefourth transmission node via wireless communication with the secondtransmission node, the third transmission node, and the fourthtransmission node, and controls a timing when the first wave transmittertransmits the first wave based on a timer value of the first timer, thesecond transmission node comprises: a second wave transmitter thattransmits the second wave; and a second wireless communicator thatincorporates the second timer, synchronizes the second timer with thefirst timer, the third timer, and the fourth timer via wirelesscommunication with the first transmission node, the third transmissionnode, and the fourth transmission node, and controls a timing when thesecond wave transmitter transmits the second wave based on a timer valueof the second timer, the third transmission node comprises: a third wavetransmitter that transmits the third wave; and a third wirelesscommunicator that incorporates the third timer, synchronizes the thirdtimer with the first timer, the second timer, and the fourth timer viawireless communication with the first transmission node, the secondtransmission node, and the fourth transmission node, and controls atiming when the third wave transmitter transmits the third wave based ona timer value of the third timer, the fourth transmission nodecomprises: a fourth wave transmitter that transmits the fourth wave; anda fourth wireless communicator that incorporates the fourth timer,synchronizes the fourth timer with the first timer, the second timer,and the third timer via wireless communication with the firsttransmission node, the second transmission node, and the thirdtransmission node, and controls a timing when the fourth wavetransmitter transmits the fourth wave based on a timer value of thefourth timer, the reception node notifies the estimator that thereception node has received the first wave, the second wave, and thethird wave, and the estimator calculates propagation time differencesbetween a propagation time of the first wave from the first transmissionnode to the reception node and propagation times of the second wave, thethird wave, and the fourth wave from the second transmission node, thethird transmission node, and the fourth transmission node to thereception node based on points in time of reception of notificationsthat the first wave, the second wave, the third wave, and the fourthwave, respectively, have been received from the reception node, toestimate the location of the reception node based on the propagationtime differences.
 15. A reception node comprising: a wave receiver thatreceives a sound wave; and a wireless communicator that incorporates afirst timer, synchronizes the first timer with a second timer built inanother node via wireless communication with the other node, andacquires reception time information indicative of a point in time ofreception of the sound wave from the first timer.